The present invention relates to a seam welding machine for continuously welding seams at overlapping edges of articles to be welded while the seams pass between upper and lower rotary electrodes effecting welding by Joule heating.
In manufacturing juice or beer cans and the like, thin sheets are bent into cylindrical forms, and the overlapping portions of these articles are overlapped in the longitudinal direction. The overlapping portions are then welded using Joule heating. The articles are conveyed as shown in FIG. 4, that is, end-to-end in a direction A indicated by an arrow.
Generally, direct current is used to weld the seams of the articles 7 while they are being continuously conveyed in their axial direction. In the conventional direct-current welding apparatus, DC current is produced by single- or three-phase, full-wave rectification or six-phase, half-wave rectification of a commercial power source. The resulting DC current is applied directly between the upper and lower rotary electrodes. However, it is unavoidable that the welding current has peaks and troughs occurring at times determined by the frequency of the commercial power source, with the relative magnitudes of the peak and troughs being affected by the time constant (L/R) of the secondary circuit of the welding machine. Accordingly, it has been impossible to change the welding current waveform instantaneously. Moreover, if a welding current peak occurs as the end portions of an article being welded are passing the welding electrodes, overwelding tends to occur in those areas, thereby yielding a generally poor welding quality.